In surface profiling, a surface contour or profile is acquired by measuring the elevation of the surface at intervals along the surface. Surface profiling methods include either non-contact methods using optical (e.g. laser) or ultrasonic transducers, or contact-based methods using ground-engaging apparatus.
Contact-based profilers are generally characterized either as the walking or the rolling type. Walking profilers include those having spaced ground-engaging “feet” or pads that are alternately brought into engagement with the surface to be measured, as the profiler is moved over a distance. An example of a walking profiler is shown in U.S. Pat. No. 7,748,264 to Prem. The majority of contact-based profilers are of the rolling type. Rolling profilers travel on wheels over the surface to be profiled. They may be manually propelled by a walking operator, or driven or towed by a vehicle. Profilers that are propelled by a walking operator, even though they may use only wheels to contact the surface to be profiled, are also commonly called “walking” profilers. Such a profiler is disclosed in U.S. Pat. No. 6,775,914 to Toom.
Walking profilers may generally be further divided into two main types. One type typically includes a frame supported on wheels and an inclinometer, pendulum or other means to measure the inclination of the entire profiler's frame. A second type generally also comprises a frame supported on wheels, and further includes one or more separate marker or sensing wheels that do not support the profiler but are connected to a transducer for direct sensing of the position of the marker wheel in relation to the supporting wheels. A relatively common prior art approach for profilers of the latter type is to provide load bearing wheels at the front and rear ends of a frame and ground-engaging sensing means mounted between the load bearing wheels. Such an apparatus is exemplified by U.S. Pat. No. 5,535,143 to Face.
A surface profiler acquires a surface contour or profile by measuring the elevation of the surface at constant distance intervals along the surface, relative to a starting elevation. Sampling the elevation in this manner produces a mathematical series of elevations, which collectively represent the physical surface along a specific line. The series can be used for a number of purposes relating to construction or ongoing management of the surface.
U.S. Pat. No. 4,741,207 to Spangler discloses a vertical distance measuring device mounted to a vehicle, which takes the form of a transducer that measures the distance to the road surface by reference to the vehicle's suspension system. However, in order for the device to produce a profile, it is first necessary to determine a stable artificial plane of reference by double integrating the signal from a vertically oriented accelerometer and then to use the distance measuring device to measure from the artificial plane of reference to the pavement. This method and apparatus describe what has come to be known as an “inertial profiler”, because of the inertial nature of the vertically oriented accelerometer sensor, which is fundamental to deriving the artificial plane of reference. In the case of low speed profilers, it is not possible to create a stable artificial plane of reference since drift inherent to the technique will invalidate the reference over the fairly long period of manual data collection. This is because of limitations of the inertial accelerometers used to measure the acceleration normal to the road surface. Vertical acceleration is caused by profile “pushing” the profiler up or down in response to horizontal movement over the profile at fairly constant speeds. If the horizontal speed is low, the vertical acceleration will be correspondingly low. At the fairly low operating speed of a walking profiler (typically about 4 km/hour, depending on the roughness of the profile), the vertical acceleration would be much less than 1 G (the acceleration of earth's gravity). Based on current accelerometer technology, this would result in a very low signal relative to noise, bias drift and other sources of error. The double integration of this weak signal would tend to yield an error value that would grow over the long profiling duration of, for example, 15 minutes required to collect data for a 1 km profile.
Various mathematical algorithms can be applied to the elevation series to calculate indices that are representative of the roughness or smoothness of the surface. The roughness relates to the discomfort that would be experienced by a passenger riding in a real or simulated vehicle that rolls over the surface. One of these indices is the International Roughness Index (IRI), which models the suspension of a nominal quarter of an automobile that is rolled over the surface within a computer model. The IRI algorithm computes the total travel of the quarter car's suspension per unit of distance traveled while rolling over the subject profile—the greater the travel, the higher the IRI value or roughness. IRI is increasingly being used for surface construction contract management. The quality of a newly constructed surface is compared to its contractual end product specification to determine if the finished surface is compliant with the specification. Construction contracts can be managed using surface profilers, with contract bonuses and penalties payable depending on profile test results. IRI is coming into use as the preferred index to determine profile quality. It should be apparent that instruments used to acquire the elevation series representing the actual surface profile that are used to calculate the IRI must therefore have high levels of accuracy and repeatability.
IRI is also being used for management of large-scale networks of roads within the jurisdictions of state departments of transport and highways, where non-contact surface profilers capable of collecting data at highway speeds are commonly being used. These are typically inertial profilers that measure elevation with reference to an inertial reference derived by double integrating the signal from a vertically oriented accelerometer. Due to their inherent limitations, such inertial profilers must be calibrated or verified against a benchmark reference or a more accurate profiling instrument to validate the data they acquire. Such benchmark devices have been defined by the United States Federal Highway Administration as “Reference Profilers”.
In recent years, research and development into roads and applications of measured road profiles has resulted in the desire for more spectral detail within the profiles. This desire arises from the interest in studying the friction and other interactions between vehicle tires and surface textural features such as may be found in longitudinal and transverse tining, longitudinally ground pavements and those pavements that use very coarse granular materials such as chip seal and stone matrix asphalt.
Low speed contact-based manual reference profilers do not use vertically-oriented accelerometers to sense vertical acceleration of the vehicle frame to derive an artificial reference plane. Instead they use inclinometers to measure the longitudinal tilting of the vehicle frame as a basis for determining the elevation of the frame. The inclinometers are typically accelerometers that measure the vector component of the earth's acceleration in the horizontal direction (orthogonal to gravity) that results when they are not perfectly horizontal with respect to the plane of the earth. This method is therefore not speed dependent.
Reference profilers must be capable of measuring fine profile features having very short wavelengths. However, prior art profiling devices employing ground-engaging wheels and inclinometers are mathematically limited to measuring only wavelengths greater than the longitudinal distance between the rotational axes of is their wheels. Specifically, inclinometer-based profilers having a frame supported by a forward wheel and a rearward wheel spaced apart by wheelbase separation distance W have the following transfer function which provides the inclinometer signal gain H at different wavelengths λ, where the straight brackets signify the absolute value of the enclosed function:
      H    ⁡          (      λ      )        =                        sin        ⁡                  (                                    π              ⁢                                                          ⁢              W                        λ                    )                                      π          ⁢                                          ⁢          W                λ                
It can easily be seen that the gain falls to precisely zero where λ=W, since sin(π)=0, and is very low for λ between 0 and W wavelength. This inclinometer-based profiler configuration is in fact an exact mechanical analog of a moving average filter having sample length of W, and the challenge presented is that the geometry of the profiler apparatus actually filters out the wavelengths that are of interest, namely those shorter than W.
It is therefore known to employ non-contact measuring devices to enhance the measuring capabilities of an inclinometer-based profiler. For example, U.S. Pat. No. 7,748,264 to Prem discloses the use of an array of laser-based height sensors attached to a frame. The frame is translated step by step longitudinally along the profile and provides a series of snapshots of the transverse profile along the longitudinal span. However, it will be noted that Prem is directed to transverse profile measurement. An inclinometer measures the transverse tilting of the frame but not the longitudinal tilting of the frame. Since no means are provided to accurately determine the longitudinal elevation differences of the frame it is not possible to derive a mathematical series that accurately represents the longitudinal elevation profile.
U.S. Pat. No. 7,044,680 to Godbersen et al. similarly discloses an apparatus to measure a surface profile using a series of paired lasers stretched transversely across the surface being profiled, attached to a moving vehicle. The frame is again translated step by step longitudinally along the profile, at relatively large 2 inch increments. The method requires fairly complex calculations and painstaking tracking of the locations of the x and y values at the front and rear of a measuring arm and thereafter careful tracking of changes in both of these values. An accurate starting datum is critical, and indeed Godbersen dedicates much of the patent specification to describing various methods for determining that information. The laser pairs are about 36 inches apart and the apparatus is therefore limited to detecting features having wavelengths of 36 inches even though the longitudinal measurement increments are shorter, because the spectral content of wavelengths in the recorded mathematical series representing the profile will still be limited to the spacing between the non-contact sensors. The transfer function of the apparatus is also described by the preceding formula for H(λ) and will be zero at the 36 inch spacing of the lasers, and close to zero at wavelengths smaller than 36 inches, making the 2 inch data sampling interval somewhat pointless in this range.
Further, the placement of the various pairs of lasers in a spaced relationship transversely across the surface being profiled leads to additional complications in the readings being taken and in the calculations necessary to produce useful data from those readings. In a system such as that disclosed in Godberson, where the distance measurement is not made at the mid-point between the non-contact sensors, an error may be introduced wherever the slope of the surface profile at the non-contact sensors differs from the slope at the location of the distance sensor. Small errors in the difference of the distance measured by the lasers become amplified in longer profiles leading to large end elevation errors. It is therefore necessary to reduce or eliminate the contribution of the non-contact sensors when measuring long distance profiles by filtering out the long wavelengths from the signals they provide, for example using high pass filtering to eliminate wavelengths longer than 10 meters, which also removes the DC (direct current) component of the signal. The inclinometer becomes the primary instrument for determining long wavelength profile features since it maintains accuracy for longer wavelengths. This can unnecessarily increase the complexity of the surface profiler.
Further, there are significant differences between the physics and geometry of inertial profilers and low speed contact-based manual reference profilers which make implementation of laser measurements to low speed contact-based profilers challenging. For example, Surface Systems & Instruments CS8800 Walking Profiler (http://www.smoothroad.com/products/walking/) is a surface profiler provided with a single lateral line scan laser attached to the frame to supplement the profile information gathered from the inclinometer. Such a single laser is generally attached to the mid-point of the frame and is directed downward so that it “sees” the ground underneath the profiler. However, the single laser is still incapable of measuring short wavelength features, and accordingly the CS8800 profiler must be provided with a contacting front arm apparatus. Further, since the frame rises and falls in response to the wheels' contact with the surface being measured, the measurement is not based on a stable plane of reference. If the distance measured by the laser is simply added to the profile generated from the inclinometer in an effort to “fill in” the missing response at distances smaller than the wheel spacing, the irregular wavelength response shown in FIG. 5 results: clearly the data obtained is inaccurate and of little practical use in the wavelength range shorter than the wheel spacing W. Consider also the situation shown in FIG. 8, where the profiler traverses a simple continuous sinusoidal profile with wavelength equal to the distance between the rotational axes of the wheels. When the profiler's wheels are resting on the crest of the sine wave the laser sees the full depth of the trough. When the profiler's wheels are resting on the trough of the sine wave, the laser sees the full height of the crest. The difference between the two gives a normalized gain of about 2 at this wavelength, representing a 100% error. At shorter wavelengths the response is erratic and overall this apparent solution is not at all useful. In summary, the distance measured by the vertical distance measuring device cannot simply be added to the profile derived by the inclinometer. Information provided by a single laser will at best be able to improve the resolution of the profiler making it capable of measuring features approximately half the size of the distance between the rotational axes of the wheels, that is, W/2.
It is therefore an object of this invention to provide a surface profile measuring apparatus that will address one or more of the issues present with currently available profilers.
It is further an objective of this invention to provide an apparatus and method to measure a surface profile continuously and with high resolution, meaning that very short wavelength profile features may be accurately identified and measured.
The present invention, given its high accuracy and repeatability, while finding uses in several industries and for many purposes, will be of particular value in both the contract management of new surface construction and as a reference standard for certification of other instruments.
These and other objects of the invention will be better understood by reference to the detailed description of the preferred embodiment which follows. Note that the objects referred to above are statements of what motivated the invention rather than promises. Not all of the objects are necessarily met by all embodiments of the invention described below or by the invention defined by each of the claims.